Characterization of Temperature Phased Anaerobic Digestion for Organic Solids Stabilization

Huoqing Ge (2011). Characterization of Temperature Phased Anaerobic Digestion for Organic Solids Stabilization PhD Thesis, School of Chemical Engineering, The University of Queensland.

       
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Author Huoqing Ge
Thesis Title Characterization of Temperature Phased Anaerobic Digestion for Organic Solids Stabilization
School, Centre or Institute School of Chemical Engineering
Institution The University of Queensland
Publication date 2011-04
Thesis type PhD Thesis
Total pages 139
Total colour pages 8
Total black and white pages 131
Subjects 03 Chemical Sciences
Abstract/Summary Modern wastewater treatment plants now produce more waste activated sludge, with low inherent treatability, due to a shift in focus towards nutrient removal rather than only sanitation. Food industry and abattoirs also produce significant volumes of waste solids. Organics in these solids streams can be readily converted to energy by anaerobic processes, which generate renewable methane. Conventional anaerobic processes require readily degradable substrates for the best outcomes. However, the degradability of long sludge-age activated sludge is normally poor, resulting in long digester hydraulic retention time (HRT), high mixing costs and poor methane production. To enhance the sludge degradability, incorporating a pre-treatment prior to conventional anaerobic digestion has been used and proven as a promising method. Since many long sludge-age activated sludge systems are small-medium scale (<5 dry tonnes solids produced per day), energy and capital intensive options such as thermal hydrolysis may not be viable in these applications. To specifically address the needs of smaller scale solids producers, this research focuses on enhanced performance with moderate thermal pre-treatment, as thermal energy is available for free when methane is converted to electricity. The process is named as “Temperature phased anaerobic digestion (TPAD)”, which consists of a biological pre-treatment digester operated at 50-70°C (2 days HRT) and a following mesophilic (35°C) anaerobic digester, which produces methane. Literature review identified that while improved performance has been observed, improvements have not been quantitatively characterised, and the mechanism by which improvement occurs has not been identified. Two two-stage lab-scale TPAD systems were set up and operated in parallel under various operating conditions with both primary and activated sludges as substrates. One was control, with the first stage at 35°C, while the other was the experiment, with the first stage operated at thermophilic conditions (50-70°C). The experimental system exhibited performance improvements of 10-30% additional VS destruction over the control system. The mechanism was identified by model based analysis, which showed thermophilic pre-treatment increased the degradation rate, but not ultimate extent for both primary and activated sludges. Methane production was also enhanced in the first stage of the thermophilic system, presumptively related to acetate oxidation. A novel two-stage batch test method was also developed and used to find the optimal temperature for the first stage. With a first stage temperature of 65°C, neutral pH and short retention time (1-2 days), the optimal TPAD performance was observed. A relative kinetics analysis using acclimatised inoculum identified that hydrolysis rates of cellulose followed the Arrhenius equation. Based on the results obtained, hydrolysis coefficients will at least triple from 0.2 d-1 to 0.6 d-1 (95% error of ~0.1 d-1), allowing substantial process intensification, positive energy balance, and mitigation of process risk through methanogenesis in the first stage. These results can be used directly by end-users through a set of design curves provided in this thesis.
Keyword Temperature phased anaerobic digestion
Thermophilic pre-treatment
Mesophilic pre-treatment
Primary sludge
Waste activated sludge
Hydrolysis rate
Degradability
Batch testing
Mathematical model
Methanogenesis
Additional Notes P31 and P72 should be printed in landscape. P16, P35, P70, P74, P78, P80, P87 and P98 should be printed in color.

 
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Created: Thu, 23 Jun 2011, 14:32:19 EST by Ms Huoqing Ge on behalf of Library - Information Access Service